Question: On the first 5 tests of his language class, Luis got an average score of 85. What does he need on the next test to have an overall average of 87?
Solution: Let his score on the next test be $x$ The sum of all of his scores is then $5 \cdot 85 + x$ The same sum must also be equal to $6 \cdot 87$ Solve: $x = 6 \cdot 87 - 5 \cdot 85 = 97$.